Percutaneous access devices hereinafter referred to as PAD, are employed to establish a connection projecting outward through the skin between an organ or a device implanted on a long term basis within the human body, and an external device such as a monitor, pump or the like. The PAD provides both mechanical and electrical access to the internal organs or devices and as such can be equipped with channels for conveying fluids or gases as well as electrical contacts for transmitting signals, for example, an ECG, or supplying power to devices. Such electrical contacts are usually partially exposed and could convey voltages and currents or become the grounding point for a substantial discharge of electricity generated in fabrics, carpets and the like.
For this reason it is necessary to install a current limiting device between the PAD contact and the organ or device, and this places the location of the limiter within the PAD (as a non-surgically renewable component). Because current limiters can fail, it is necessary that the limiters are positioned such that removal and replacement is possible without resorting to any surgery. Electrical contacts as well as fluid sealing surfaces and mechanical connection features also require periodic monitoring and maintenance since they are subject to everyday abrasion, abuse and actual breakage. The placement of electrical and/or mechanical devices in the PAD housing complicates the task of performing routine diagnostics and general troubleshooting and is limited by the permanent connection of the PAD to organs and devices requiring a carefully controlled operating or interrogating procedure.
A turret assembly containing the current limiters, electrical contacts and the fluid coupling and sealing device is used to render all of the components that are susceptible to breakdown and damage (except the implant body itself) to rapid removal and replacement by a fresh turret so that the PAD function is only momentarily disabled.
It is physiologically advantageous to use a PAD size consistent with sound implantation practices (minimal intrusion) and the ability to survive intact for long periods of time (conservative material mass). This severely limits the space and location available for a turret and requires the use of a small screw fastener at the bottom of the turret which must be accessed by a screwdriver working through the required air inlet passage which branches off just shy of the bottom.
It is desired to make the installation and lock down or removal of the turret as foolproof and rapid as possible, capable of accomplishment by a person of average skill. It is further desired to preclude a person from using tools or fasteners that are unsuitable. Further, it is desired to prevent a screwdriver of the wrong shape or size being applied to the screw head so that the drive slot or socket is damaged preventing removal of the turret. It is desired to prevent the fastener being overtorqued such that it is structurally compromised. It is desired to prevent the fastener prior to tightening, to shift in orientation or assume a position which makes the screwdriver engagement difficult or impossible. It is desired to prevent the wrong fastener being used which can jam into the threaded receiver of the PAD body and making turret removal very difficult. Further, it is desired to prevent excessive torque being applied to the fastener causing trauma to the PAD or skin interface.
A situation akin to the PAD example above, in which a turret or insert having a slenderness ratio of approximately 3 to 1 (length to diameter) or greater, is frequently encountered in mechanical and electromechanical assemblies. Oftentimes such a component is held into a base by means of a threaded fastener because it is only a threaded fastener that will provide great holding strength while occupying the smallest possible volume that makes lockdown of the part possible. Screws are also among the most economical of fasteners, are universally understood and are easily replaceable.
The installation and removal of such turrets is complicated by the difficulty of positioning the fastener drive tool so that it will engage the screw slot or drive socket. Visibility is usually very limited and may even be unattainable for deep turrets with small passages that access the screw, where the driver occupies most of the passage. Furthermore, the screw may be quite short in length and have a small head and not fall into the pilot diameter or clearance hole in the bottom of the turret. The turret may be a drawn can or a part with a thin bottom and simply having a clearance hole in lieu of any pilot hole. It can then become very difficult to probe for the fastener to get it to fall into place ready for driving. All this can become substantially more challenging if time is of the essence (a medical procedure), or if one is working upside down, for example, because the base (which could be a vehicle underside) cannot be strategically placed in a comfortable working position.
For these reasons, devices such as captive fasteners are sometimes used where the screw is disengaged from the base by turning but which cannot fall out of the turret clearance or pilot hole. The captive fastener is not an option for many assemblies due to considerations of space, cost and strength. These captive fasteners can still misalign because they are inherently free to mutate in the clearance hole. One common variant of such fastener devices involves relieving the threads adjacent to the head so that the screw may be threaded into a tapped hole in the bottom of the turret and fall into place and be captivated when the relief diameter is reached. This fastener scheme is limiting in application because it is usually desired to push the turret into the socket or housing prepared for it in the base so that it bottoms out and in some cases keys in place for the proper orientation. To accomplish this with a relieved screw requires that the screw be able to slide up into the turret so it does not interfere with seating or that it fit into a counterbore in the housing for the same reason. Both solutions mandate more depth of material and lead to increasing the overall size of the assembly. In the case of very tiny screws, i.e. No. 1 and smaller, thread relief becomes a difficult and expensive option because most screws are produced by thread rolling and will require post machining.
Yet another option is to push the turret into position and screw it down by holding the screw in a locking or magnetized (ferrous screws) driver. Such drivers frequently fail to hold the fastener properly leading to cross threading and possible thread damage which goes unnoticed and can lead to product failure or make removal a daunting task. The screw can also be dropped accidentally into the turret hole and may be very hard to remove (non-magnetic materials).